The phosphorus in phosphate rock or in its concentrates, e.g. apatite and phosphorite, is insoluble or only sightly soluble in water and thus available for plants in limited quantities.
In producing phosphorus fertilizers the objective is to increase the solubility of phosphorus.
Normally the solubility of phosphorus contained in the fertilizer is defined as a proportion of the soluble phosphorus, soluble either in water, in ammonium citrate or in 2% citric acid, compared to the total phosphorus content.
The most common production of phosphorus fertilizers is to treat phosphate rock with an inorganic acid, producing a compound suitable for fertilizing purposes as such, or for further processing.
Another method for producing phosphorus fertilizers is the heat treatment of a mixture of phosphate rock and some other substances at a high temperature.
Phosphorus fertilizers are also obtained from iron ores containing phosphorus, as a by-product in the steel industry. The phosphorus fertilizers are produced by a reaction of phosphate rock with an inorganic acid.
When sulfuric acid is used for the reaction with phosphate rock, the reaction product is a mixture of monocalcium phosphate and calcium sulphate (gypsum), called superphosphate.
______________________________________ [Ca.sub.3 (PO.sub.4).sub.2 ].sub.3 CaF.sub.2 + 7H.sub.2 SO.sub.4 -- Phosphate rock Sulphuric acid (Flourine apatite) 3Ca(H.sub.2 PO.sub.4).sub.2 + 7CaSO.sub.4 + 2HF monocalcium calcium hydrogen phosphate sulphate flouride ______________________________________
The superphosphate contains approximately 8% phosphorus, of which at least 93% is soluble in water.
When the tricalcium phosphate in phosphate rock is allowed to react with sulphuric acid and water, the reaction products are phosphoric acid and calcium sulphate: EQU Ca.sub.3 (PO.sub.4).sub.2 +H.sub.2 SO.sub.4 +6H.sub.2 O.fwdarw.3CaSO.sub.4 .multidot.2H.sub.2 O+2H.sub.3 PO.sub.4
Phosphoric acid is used in a further reaction to produce phosphorus fertilizers, such as triple superphosphate and ammonium phosphates.
In a reaction of phosphate rock with phosphoric acid, triple superphosphate, an almost pure monocalcium phosphate compound, is produced: EQU [Ca.sub.3 (PO.sub.4).sub.2 ]3.multidot.CaF.sub.2 +14H.sub.3 PO.sub.4 .fwdarw.10Ca(H.sub.2 PO.sub.4).sub.2 +2HF
The phosphorus content of the triple superphosphate is approximately 22%, of which at least 93% is soluble in water. The reaction product of phosphate rock and hydrochloric acid is dicalcium phosphate (CaHPO.sub.4), which has little value as a fertilizer.
The reaction of phosphate rock with nitric acid produces a mixture of monocalcium phosphate and calcium nitrate.
After calcium nitrate has been removed, the mixture contains monocalcium and dicalcium phosphates. The N:P ratio in the fertilizer is generally in the range of 1:0.44 to 1:1.3, and its content of soluble phosphorus is 0 to 80%, depending on the production method.
Phosphorus fertilizers manufactured by heat processes include Rhenania phosphate, produced by heating a mixture of phosphate rock, sodium carbonate and arenaceous quartz at a temperature of 1200.degree. C. EQU Ca.sub.10 (PO.sub.4).sub.6 F.sub.2 +3Na.sub.2 CO.sub.3 +2SiO.sub.2 +H.sub.2 O EQU 6CaNaPO.sub.4 +2CaSiO.sub.4 +3CO.sub.2 +2HF